1. Field of the Invention
The present invention concerns a variable moment vibrator usable in particular, but not exclusively, for driving objects such as piles and sheeting piles into the ground.
2. Description of the Prior Art
Vibrators routinely used in this kind of application employ at least one pair of rotating eccentric weights and means for rotating their drive shafts at the same speed in opposite directions.
It is clear that with such arrangements the centrifugal forces generated by the rotation of the weights add in a direction defining a working axis and compensate each other in other directions, cancelling out in a direction perpendicular to the working axis.
For many reasons it is desirable to be able to adjust the amplitude of the vibrations generated by the vibrator, for example to allow for the mechanical characteristics of the soil, and to obtain the optimum efficiency.
The first solution that comes to mind for carrying out such adjustment is to vary the rotation speed of the weights using variable speed drive means. However, in this particular field of application variable speed drive means (usually hydraulic motors) are bulky, often too costly and possibly too fragile so that in practise this solution is not used.
Another drawback of conventional vibrators (also found in variable speed vibrators) results from the fact that on starting up the speed of the weights increases progressively to the nominal speed and during this period the speed passes through critical values related to resonant frequencies of the mechanical system. The resulting transient phenomena may damage the components. The same phenomena occur when the vibrator slows down on being turned off.
Another solution, proposed in U.S. Pat. No. 3,564,932 is to use a structure comprising at least two series of weights each comprising at least one pair of eccentric weights rotating in opposite directions, using a Pecqueur epicyclic gear to achieve an angular phase-shift between the two series of weights. This solution is ruled out because of the excessive gearing that it requires and because of the resulting drawbacks with regard to cost and problems of wear. It has never been put into practise.
Other solutions disclosed in the application WO-A-8 907 988 or in the Japanese application JP-A-59 177 427 propose coupling coaxial eccentrics by means of a rotary linkage using two rotary members movable axially relative to each other against the action of a spring by a pressurized fluid. One of these members comprises a helical groove and the other comprises a finger inserted in the groove so that axial displacement of one part relative to the other causes relative rotation of the two parts.
It is found that this solution has a number of drawbacks.
Firstly, the mechanical finger/groove coupling employed cannot be used in a vibrator because of the very small dimensions of the surfaces of contact between the finger and the groove. For this reason the phase-shifter is unable to withstand the vibrations produced by the vibrator.
This drawback is all the more accentuated if the phase-shifter is directly coupled to the eccentric weights and so is subjected to high stresses (resulting from the centrifugal forces generated by the eccentric weights, which can exceed ten tons).
Another drawback of known systems is that they provide no way of adapting the vibrational power transmitted to the working conditions of the tool to which the vibrations are applied and to the characteristics of the power source.
A particular object of the invention is to eliminate these drawbacks.